The vertical distribution of scalants as a result of 24 h of filtration of simulated low-level radioactive liquid waste in the reverse osmosis (RO) membranes was characterized by means of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Our results suggested that the development of scalants is associated with the formation of metal hydroxide colloids, while these colloids might penetrate relatively deeper into the membrane under the acid environment than under the alkaline environment. Fitting with a two-phase fouling Hermia's model suggested that the development of cake-like scalants in the first 5 h under alkaline environment is responsible for hindering the assess of metal hydroxides into the membrane so that they would not penetrate as deep as they do under the acid environment. (C) 2014 Elsevier B.V. All rights reserved.